1. Field of the Invention
This invention relates to a focus detection device used in an optical instrument such as camera or the like, and more particularly to a so-called active-type focus detection device in which light is projected from the device side to an object and the light then reflected by the object is photoelectrically detected to thereby detect the focus adjusted condition of an image-forming optical system with respect to the object.
2. Description of the Prior Art
As the system for photoelectrically detecting the focused position of an image-forming optical system with respect to an object, there has long been known the system whereby the non-linear response characteristic of photoelectric substance is utilized to detect the contrast of the image formed by the optical system or the system whereby a lenticular screen or the like is disposed in the prearranged focal plane of the optical system and a photoelectric cell is disposed behind the lenticular screen or the like, so that the focus or the non-focus of the optical system is detected by detecting whether the output from the photoelectric cell becomes a direct current or a pulsating wave during rotation or vibration of the lenticular screen (this is proposed, for example, by U.S. Pat. No. 3,450,018). Both of these systems utilize the light signal produced from the object itself and are called the passive-type focus detection system. The greatest disadvantage of such type of system is that the detection performance thereof is greatly affected by the light signal from the object so that the focus detection is very difficult, for example, for objects of low contrast or of low brightness.
In contrast, there is also known the so-called active-type focus detection system whereby light is projected upon an object and the reflected light therefrom is photoelectrically detected to thereby determine the focused position of the optical system. According to this system, which utilizes an artificial light signal emitted toward the object, the above-described disadvantages peculiar to the passive-type detection system can all be overcome.
However, the following disadvantages exist in the heretofore proposed device adopting the active-type detection system.
As an example of the device of this type, U.S. Pat. No. 3,435,744 or U.S. Pat. No. 3,442,193 proposes a base line range finder type active focus detection device for camera which utilizes the principle of the base line range finder and in which a projected and a photoelectric light receptor are disposed at a predetermined base line interval and at least one of the projector and the photoelectric light receptor is movable and disposed in interlocking relationship with a picture-taking lens or a movable mirror is disposed in the light-receiving path of the photoelectric light receptor in interlocking relationship with the picture-taking lens, and the picture-taking lens is suitably adjusted when the light projected from the projector and reflected by an object is detected by the photoelectric light receptor so that the picture-taking lens is properly focused with respect to the object when the photoelectric light receptor comes to properly receive the reflected light from the object. In the device of this type, however, some interlocking mechanism between the picture-taking lens and the focus detection system, namely, the mechanism for varying the condition of incidence of the reflected light upon the photoelectric light receptor in accordance with the adjusted position of the picture-taking lens, is indispensable and this makes complicated the mechanical construction of the small compact instrument such as camera or the like into which the mechanism is to be incorporated, and in fact, the incorporation thereof into the camera or the like involves much difficulty. Also, in this case, if a mechanical error is created in this interlocking mechanism, such error will greatly affect the detection accuracy to deteriorate the detection accuracy remarkably. Particularly, in the device of this type which utilizes the principle of the base line range finder, a predetermined length of base line must be set between the projector and the light receptor and the greater the length of base line, the easier the focus detection and the higher the accuracy, but a greater length of the base line results in larger size of the entire device and particularly in a small instrument such as camera or the like, the settable length of the base line is limited and accordingly, the improvement of the performance is limited.